Large induction motors are required for many heavy duty applications, such as driving air conditioning compressors for large buildings. These motors must deliver in excess of 900 horsepower at 3600 rpm, and also must operate properly at low speeds under 10 rpm. In the case of D-flange motors, the compressor is connected directly to the motor housing, limiting access to the bearings after installation. Therefore, lubrication of the motor bearings in these applications has been an area of great concern.
Grease lubrication has been used in many large motors, because of the difficulty in maintaining a film of oil in large sleeve bearings operating at low rpm. However, when bearings are lubricated with grease and operated at high rpm, the grease begins to churn and becomes overheated, leading to break down of the lubricant. Sleeve bearings on fairly large motors have been oil lubricated using an oil mist system or oil distribution rings which dip into an oil sump and then deliver oil to the bearing. However, sleeve bearings require very accurate alignment and therefore are not desirable in D-flange motors.
The rotor of an induction motor is typically positioned on the motor shaft within an interior space enclosed by the stator windings and the bearing support structure. Motor bearings are normally captured between a bracket defining a cavity for receiving the bearing, and a cap which holds the bearing in the bracket. Bearing seals also held by the cap or bracket are positioned on either side of the bearing to retain lubricant and exclude contaminants. It is known to utilize protrusions or blades extending from the ends of the rotor as a fan to draw cooling air into the interior and to distribute the air over and between the interior parts. The action of these protrusions reduces the pressure within the interior space between the rotor end and the bracket, and may cause the bearing lubricant to be pulled from the bearing structure and seals into the inner portions of the motor. Lubricant contacting the windings of the motor tends to swell and deteriorate the insulation, and eventually shorts out the motor, causing it to fail. Other problems associated with loss of lubricant may occur in other rotary devices which include bearings.
Various attempts have been made to equalize the pressure on both sides of bearings in order to prevent lubricant leakage. Hoses have been connected between the inner side of the bearing and the exterior wall of the housing or bearing support structure, but it is very difficult to connect the hoses while assembling the bearing. U.S. Pat. No. 4,039,229 discloses a grease-lubricated bearing with a flexible membrane enclosing the grease chamber. A passageway through the motor housing from the membrane to atmosphere allows the membrane to flex, thereby relieving any pressure increase within the grease chamber. U.S. Pat. No. 3,466,478 discloses spaces on either side of a ball bearing, connected by passages formed in the bracket. The pressure drop normally present is made to occur at restricted orifices between the spaces and the air adjacent to both sides of the bearing.
Passageways through bearing support structures for the purpose of conducting lubricants are shown in U.S. Pat. Nos. 4,844,625 and 5,001,377. Such passageways do not open to the interior space within the motor, in order to avoid pumping lubricant to undesirable areas. U.S. Pat. No. 2,210,705 shows a ball bearing immersed in an oil bath.
Despite prior efforts, there has remained a need in the art for a high speed motor with oil lubricated ball bearings and a system for venting the space on the inner side of the bearings to ambient atmosphere without requiring any extra steps in assembling the bearing.